The Lake’s iconic transparency and stunningly blue waters are often the first thing that comes to mind when people think of the Tahoe Region. The lake’s clarity has been regularly measured since 1968, when UC Davis first started lowering a Secchi disk into the lake, establishing one of the longest, unbroken clarity measurement records in the world. The Secchi depth measurements are perhaps the best known of all the indictors of the Region’s environmental health. Between 1968 and 2000, the clarity of the lake regularly declined, but the long term decline in clarity observed in the end of the 20^th century appears to have been halted about 20 years ago. While this is an encouraging trend, there is still much work to be done, and is but one of many measures of the health of Region’s aquatic systems.

The Bi-State Compact requires the Regional Plan to provide for the attainment and maintenance of federal, state, and local water quality standards. Resolution 82-11 sets out numerical standards, management standards, and policy statements for water quality. Some of these threshold standards are referenced to state standards. In other cases, Resolution 82-11 sets targets based on  reference conditions related to specific periods, and can be found in the Study Report for the Establishment of Environmental Threshold Carrying Capacities (TRPA, 1982). The value statements were developed in 1982, that guided the development of threshold standards for water quality were:

• Attain levels of water quality in the lakes and streams within the basin suitable to maintain the identified beneficial uses of Lake Tahoe.
• Restrict algal productivity (rate of growth) to levels that do not impair beneficial uses or deteriorate existing water quality conditions in the Lake Tahoe basin.
• Prevent degradation of the water quality of Lake Tahoe and its tributaries to preserve the Lake for future generations.
• Restore all watersheds in the basin so that they respond to runoff in a natural hydrologic function.

Prior to the arrival of European settlers, fire, floods, and other natural disturbances (e.g., earthquakes, landslides, or avalanches) were the major drivers of pollutants like fine sediments and nutrients entering the lake. However, these were likely episodic in nature, with potentially substantial intervening periods between major events. More regular, low-intensity fires and a mature forest likely translated into low-nutrient stores on the forest floor. These were the watershed conditions that supported an ultraoligotrophic Lake Tahoe: a lake with a sustained level of exceptional water clarity (greater than or equal to 30 meters), a lake receiving low inputs of nutrients and therefore supporting low levels of primary productivity, and a lake containing a relatively simple food web that may have substantially relied on the recycling of nutrients and carbon, rather than new inputs from the surrounding watershed.

Urbanization and development altered the natural hydrologic regimes of many of the catchments in the Tahoe Region. Studies completed as part of the Lake Tahoe Total Maximum Daily Load (TMDL) show that urban areas are the primary source of fine sediment (the pollutant known to impact lake clarity). Much of the urban development has occurred along the edge of Lake Tahoe, meaning that in many cases, there is little or no buffer between the source of pollution and the Lake. The concentration of development also represents an opportunity for managers in the Region to mitigate impacts.